Most GPS and Supersonic Speeds Don’t Mix
As if the cost of the zero pressure balloons is not enough, we have a real burden in using a GPS system that works at supersonic speeds and also one that worked above 60,000 feet. Yes, we want to do both so a standard GPS system will not work.
So Why the Limits?
It is not such a big issue these days as technology has moved on, but 10 to 15 years ago, this was a major deterrent to anyone wanting to use them in a missile. Unfortunately today, that is less of a deterrent as most people could easily source someone capable of updating the GPS firmware. None the less it is still a better and safer path to pay the manufacturer for a GPS system that has the limits removed. There are many manufacturers that can provide the product and some are harder to work through than others. This is often the result of a country’s regulations regarding export licenses. For instance, buying from Canada is simpler than buying from the US. It still takes a couple of weeks, but the opportunity to get a limited version for testing will allow us to swap out the limited version for the unlimited version prior to flight. As the system that we wish to use is available in Australia from a local distributor, it is very likely that we will buy from a company called NovAtel. Their product is the Receivers OEM615, although they have more expensive products that would do much more for us.
What are the Limits on Regular GPS Engines?
This from Wikipedia: CoCom is an acronym for Coordinating Committee for Multilateral Export Controls. CoCom was established by Western bloc powers in the first five years after the end of World War II, during the Cold War, to put an arms embargo on COMECON countries. CoCom ceased to function on March 31, 1994, and the then-current control list of embargoed goods was retained by the member nations until the successor, the Wassenaar Arrangement, was established.
In GPS technology, the phrasing “COCOM Limits” is also used to refer to a limit placed to GPS tracking devices that should disable tracking when the device realizes itself to be moving faster than 1,000 knots (1,900 km/h; 1,200 mph) at an altitude higher than 60,000 feet (18,000 m). This was intended to avoid the use of GPS in intercontinental ballistic missile-like applications. Some manufacturers apply this limit when both speed and altitude limits are reached, while other manufacturers disable tracking when only a single limit is reached. In the latter case, this causes some devices to refuse to operate in very high altitude balloons.
Can we get a Single Limited GPS Engine?
It is hard, but it is not impossible. If it was not for the manufacturers implementing an “or” function instead of an “and” function we could possibly manage to use a unit that would measure our speed and display GPS co-ordinates at over 60,000 feet (18km provided that our speed was under 1,900kph / 1,200mph). This is difficult as we may go over that speed limit. At that point the GPS output is usually nulled. We need data at all times and do not want a blackout on our data. It will also let us say that we broke the sound barrier, but not by exactly how much. Thus we want a a fully unlimited module for the flight.
Ideally we would like to store our flight measurements. We will have telemetry and can store everything that is down-linked, but there is a risk in doing that. If we use a more expensive unit, then we can have “on-board ” storage. This is mission critical if the telemetry link malfunctions. The aircraft will still fly itself to the runway on auto navigation and we can try again if we have the gas and a second balloon.
Another issue is the rate of poling of the GPS data. We need more than once a second or we could miss our top speed by hundreds of kilometers and hour. This means simply poling of maybe 20 times a second. This rate is easily supported by our telemetry so we will get an instant top speed on the ground before it lands. Something that a basic unit will not be configured to do.
Specifications for the NovAtel Unit.
This is not the one that records, nor is it the top of the vibration resistant unit, but it is very well placed to do the job, after all, we can record the data on other equipment before it is sent to the telemetry system. The cost of the export License is probably $5K and the cost of the unit will be another $5K making a grand total of $10K. The Export License checks your usage of the device and makes sure that you are not a group building a missile for nefarious reasons.
The following from NovAtel’s documentation:
The dual-frequency OEM615 offers future ready, precise positioning for space constrained applications. Backward compatible with NovAtel’s popular OEMV-1 form factor, the OEM615 provides the most efficient way to bring powerful Global Navigation Satellite System (GNSS) capable products to market quickly.
- Increased satellite availability with GLONASS tracking
- L1, L2, L2C, B1 and E1 signal tracking
- GLIDE smoothing algorithm
- RT-2®, ALIGN and RAIM firmware options
- SPAN® INS functionality
- Proven NovAtel technology
- Easy to integrate
- Low power consumption
- API reduces hardware requirements and system complexity
Number of Com Ports
|Single Point L1||1.5m|
|Single Point L1/L2||1.2m|
|RT-2®||1 cm + 1 ppm|
Designed with Performance and the Future In Mind
The OEM615 tracks all current and upcoming GNSS constellations and satellite signals including GPS, GLONASS, Galileo, BeiDou and QZSS. It features configurable channels to optimize satellite availability in any condition, no matter how challenging. The OEM615 is software upgradable to track future signals as they become available. Maximizing satellite availability and optimizing GNSS signal usage now, and in the future, ensures consistent, high performance GNSS positioning.
– See more at: http://www.novatel.com/products/gnss-receivers/oem-receiver-boards/oem6-receivers/oem615/#sthash.caG8JrgA.dpuf
Dimensions 46 × 71 × 11 mm
Weight <24 g
Input voltage +3.3 VDC ±5%
GPS L1/L2 <1.0 W
GPS/GLONASS L1/L2 1.1 W
all on 1.2 W
Antenna LNA Power
Input voltage 6 VDC-12 VDC
Output voltage 5.0 VDC
Max output current 100 mA
3 LVTTL up to 921,600 bps
2 CAN Bus12 1 Mbps
1 USB 12 Mbps
Pulse Per Second (PPS) output
Operating -40°C to +85°C
Storage -55°C to +95°C
Humidity 95% non-condensing
Random MIL-STD 810G
(Cat 24, 7.7 g RMS)
Sinusoidal IEC 60068-2-6
Bump ISO 9022-31-06 (25 g)
Shock MIL-STD-810G (40 g)
Survival (75 g)